In recent years, with progresses of digital technologies, electronic devices such as portable information devices and information home electric appliances have been developed to provide higher functionalities. With achievement of the higher functionalities of these electronic devices, non-volatile memory devices incorporated into the electronic devices have been developed to achieve a greater scale, higher-dense integration and a higher speed, and uses of the non-volatile memory devices have been expanding at a high pace.
Among the non-volatile memory devices, a memory device has been proposed, in which non-volatile variable resistance elements are used as memory elements and are arranged in matrix. It is expected that by forming a three-dimensional memory in which matrixes are stacked together, a greater scale, higher-dense integration and a higher speed of the memory device, are attained.
The variable resistance element has a thin layer made of a material comprising, for example, a metal oxide. By applying an electric pulse to this thin layer, its electric resistance value changes and the changed electric resistance value is preserved. By corresponding a high-resistance state and a low-resistance state of the thin layer with, for example, binary data “1” and “0”, the binary data can be stored in the variable resistance element.
As the variable resistance element capable of storing the binary data, there are a variable resistance element (unipolar variable resistance element) which changes its resistance value by application of electric pulses of voltages which have the same polarity and are different in magnitude from each other, and a variable resistance element (bipolar variable resistance element) which changes its resistance value by application of electric pulses which are different in polarity from each other.
Typically, the unipolar variable resistance element has a characteristic in which write time taken to change the element from a low-resistance state to a high-resistance state (so-called reset) is longer than write time taken to change the element from the high-resistance state to the low-resistance state (so-called set). By comparison, in the bipolar variable resistance element, typically, write is enabled to occur in a short time in both of the set and the reset, and therefore a higher-speed operation is attained.
There is also known a memory device (cross-point memory device) in which a plurality of word lines cross (three-dimensionally cross) a plurality of bit lines, respectively, in a plan view such that the word lines and the bit lines do not contact each other, and variable resistance elements are placed at three-dimensional cross-points of the word lines and the bit lines, respectively. In the cross-point memory device, for example, when data is written to the variable resistance element, a leak current flowing through a cell other than a selected cell occurs. When the data written to the variable resistance element is read, a current flowing through the selected cell and the leak current flow as a bit line current to a bit line decoder, which determines a write state of the selected cell by a transistor within the decoder. If the leak current is great in magnitude even in a case where the current flowing through the bit line has an equal value, the current flowing through the selected cell is relatively small in magnitude, while if the leak current is small in magnitude even in a case where the current flowing through the bit line has an equal value, the current flowing through the selected cell is great in magnitude. In the former case, the current flowing through the selected cell is small in magnitude, which causes data to be written inadequately to the variable resistance element. To avoid this, it is necessary to lessen the leak current to ensure the current flowing through the selected cell. In addition, the leak current causes a failure (hereinafter this failure will be referred to as “write disturb”) to occur, in which a resistance state of the variable resistance element in a cell other than the selected cell changes. It is therefore necessary to prevent the write disturb. For the above stated reasons, in the cross-point memory device, in some cases, a current steering (controlling) element is provided in each cell such that the current steering element is connected in series with the variable resistance element.
The unipolar variable resistance element is able to cause resistance change in response to the electric pulses with the same polarity. Therefore, in the cross-point memory device incorporating the unipolar variable resistance element, a unipolar (single-polarity) current steering element such as a p-n junction diode or a Schottky diode (element which has a non-linear [in which its resistance value is greater in a range (lower-voltage range) in which an absolute value of the voltage is smaller and its resistance value is smaller in a range (higher-voltage range) in which an absolute value of the voltage is greater] current characteristic with respect to one of positive and negative voltages and in which its resistance value is greater and which does not substantially flow a current with respect to the other voltage), is placed in series with the variable resistance element. Thus, occurrence of the write disturb can be prevented.
Patent Literature 1 discloses a memory cell in which a variable resistance element is connected in series with a Schottky diode. With this configuration, it becomes possible to lessen an influence caused by a read disturb in a memory cell including a series circuit of the variable resistance element and the diode.
By comparison, in the bipolar variable resistance element, two kinds of electric pulses which are different in polarity are used to write data to the variable resistance element. For this reason, a bidirectional current steering element (element which has a non-linear [in which its resistance value is greater in a range (lower-voltage range) in which an absolute value of the voltage is smaller and is smaller in a range (higher-voltage range) in which an absolute value of the voltage is greater] current characteristic with respect to both of positive and negative voltages) is placed in series with the variable resistance element. As the element having such a characteristic, there are known, for example, a MIM (Metal-Insulator-Metal) diode, a MSM (Metal-Semiconductor-Metal) diode, or a two-terminal element such as a varistor.
Patent Literature 2 discloses a memory element array composed of memory elements each of which is configured as a series circuit of a variable resistance element and a current steering element connected in series with the variable resistance element (paragraph 0129). The current steering element is configured to include a current steering layer and a pair of opposing electrodes between which the current steering layer is interposed. This configuration is the same as that of the MIM diode or the MSM diode (paragraph 0050).